This invention relates generally to forming of resin-impregnated cloth, and more particularly to forming improved resin-impregnated fiberglass sheets and the resultant sheets which are specially adapted for use in forming chip carriers and the like, and which have improved resistance to generating dust particles during forming operations.
Resin-impregnated fiberglass sheets are commonly used in the formation of printed circuit boards. The fiberglass cloth is typically impregnated with the selected thermoset resin which is then partially cured and the impregnated cloth sheared to form what are known as sticker or prepreg sheets. In order to enhance the adhesion of the resin to the fiberglass, often a coupling agent, such as a silane, is coated onto the surface of the fiberglass prior to impregnation. The prepreg sheets are then laid up with sheets of metal such as copper or copper-invar-copper (CIC) and laminated with heat and pressure to fully cure the laid-up laminate with the metal sheets defining ground, power and signal planes. One of the desirable characteristics of the resin impregnated fiberglass sheets is that the coated resin must cover the fibers of the fiberglass and must be able to be partially cured to a non-tacky state wherein the sheets can be handled for the lamination process. This is often referred to as B-stage, a cure state which allows the sheets to be sufficiently self-supporting to be laid up as a laminate, but not advanced enough in the state of cure that they are rigid or non-flowable when heated, and they can be further cured to C-stage, a state of full cure, with heat and pressure to form a laminate structure as is well known in the art. As indicated above, this lamination process normally includes the lamination of one or more sheets of metal, such as copper, CIC or other metal, to provide necessary ground planes, power planes and signal planes buried within the laminated circuit board. Also in conventional practice, openings are formed, either by drilling or other means, through the fully cured laminates which form the openings for vias or plated through holes where the connections can be made from one surface of the circuit board to the other and to the various internal planes within the laminate as required.
A conventional technique of forming the resin-impregnated fiberglass sheets is to provide a coil of the fiberglass material and unwind the fiberglass material from the coil and continuously pass it through a tank containing the solution of the desired resin in a solvent, and then pass the coated or impregnated material through a treater tower wherein heat is applied to drive off the solvent and to partially cure the resin material by initiating cross-linking and then coiling the partially-cured or B staged material into a coil. Thereafter, the partially-cured material is uncoiled and cut into sheets of the desired length. These sheets, known as prepreg sheets, are then used in the lamination process described above.
This prepreg material has long been used for manufacturing circuit boards; however, more recently, the same prepreg material and same laminating techniques that have been used to form a circuit board have been used to form chip carriers. A chip carrier is basically a small-size version of a circuit board where the metallurgy and the lay out can be much finer than on a circuit board. Printed circuit board reliability tests are defined, e.g., in IPC specifications, whereas chip carrier tests are defined by JEDEC specifications which are derived for ceramic carriers and are more severe tests. During many of these manufacturing, or forming operations, such as slitting, trimming, drilling or punching of registration slots or bending handling during lay-up, the resin may delaminate from the glass fibers which in turn can generate dust particles of the resin. The dust particles which settle on top of the metal foils during lay-up into cores or composites, melt and cure during lamination, forming spots which are very resistant to chemical processing or cleaning. In chip carriers with fine line circuitry even small amounts of dust of small diameter can cause problems with forming the copper circuitry. These problems are especially pronounced where substractive etching of full surface plate metallurgy is performed to form electrical circuits or traces. Briefly, small amounts of dust can interfere with the etching process, and even xe2x80x9cclean roomxe2x80x9d procedures during lay-up and lamination do not totally eliminate such problems. Thus, it is desirable to provide a sticker sheet having improved resistance to generating dust.
It is thus an object of the present invention to provide laminated glass fiber reinforced structures and a method of making the same which structures are utilized for chip carriers which have a reduced propensity to generate dust particles during forming operations.
According to the present invention, a method for coating cloth, especially fiberglass sheets, with a resin and resulting structure is provided. The coating is performed in two steps. In the first step, essentially all of the strands of the fiberglass are coated with the resin solvent mixture, and wherein some and preferably most interstices or openings are free of the solvent resin mixture. This first coating is then partially cured to the extent that it will not redissolve in a second coating of the same resin solution and also upon heating to cure the coating the first coating is at least 70% cured, and up to 90% cured and preferably more than 80% cured to form a good adhesive bond to the glass fibers. The coated fiberglass with partially cured resin thereon is then given a second coating preferably of the same resin mixture which coats the first coating and fills in any unfilled interstices between the fibers. In some embodiments a different second resin may be employed. This second coating is then partially cured (e.g. 20% to 40% cured), which advances the cure of the first coating (to over 80% cured and preferably to 90% to 100% cured) and results in an impregnated fiberglass cloth structure for use as sticker sheets. During fabrication, the first coating which has been significantly, but not fully cured is strongly adherent to the glass fibers thus reducing the probability of breaking away from the fibers and forming dust particles during fabrication. The second coating is fluid enough to fill in spaces or interstices and only partially cured so as to form the adhesive bond to other sticker sheets and conductive layers during subsequent lamination.